In recent years, the number of full-color images is increasing in the field of electrophotography. In full-color images, an image is formed of a larger number of pixels compared to text images. Therefore, images have a tendency to have more regions of so-called solid image. Since a larger amount of toner passes through the fixing apparatus while fixing an image having such solid regions, generally silicone oil has been utilized to prevent offset when a fixing apparatus having a contact member is used. Although offset is reduced by employing silicone oil, silicone oil may remain on the image surface to cause glare or difficulty in additional writing on the image. Further, when silicone oil remains unevenly on the image surface, the image quality may be deteriorated due to the unevenness of the image.
To overcome this problem, a release agent, typically a wax, is incorporated to the toner to make silicone oil unnecessary when the image is fixed. Namely, an oil-less fixing method has been employed these days. However, in this method, as a result of toner being continuously subjected to a mechanical load due to friction with the contact member such as an image holding member or a development sleeve, the toner is crushed and exhibits a significantly different shape from the shape of the initial toner, which may cause unfavorable influence on the processes of electrostatic charging, developing, transferring and fixing.
However, since a transport device employing a contact member is a simple and efficient transport means, it may be difficult to be replaced with other methods. Accordingly, at present, a method to provide stable images using a full-color image formation method has not been fully established.
Further, solution of the above-described problem has become more important for a so-called polymerized toner than for a pulverized toner, because, even for the polymerized toner which has recently been widely employed, oil-less fixing is becoming a main current as a fixing method, since addition of a release agent in the production process is easier for the polymerized toner.
Also, in view of a desire to conservation of resources and energy, a fixing process, which consumes the largest energy among electrophotographic processes, is expected to carry out at a lower temperature and in a simple operation. In this point of view, the above-described oil-less fixing method is advantageous for a low temperature and simple operation.
In the above-described background, mechanical durability of toner is deteriorated in a long term usage because of mechanical stress at the time of development in a non-magnetic single-component development method and mechanical stress by high speed mixing in a high speed two-component method, and as a result, particularly, induced are problems of increase in a smaller particle size component due to crushed toner and offset or adhesion thereof on such as a sleeve, whereby significant deterioration of image quality is caused.
Recently, it has been confirmed that crush of the toner occurs at the interface between resin and a release agent, and that this phenomenon is particularly significant in a development method employing a low temperature fixing toner having a tendency of softening and in an oil-less toner which utilizes more amount of a self-contained release agent, which is in well coincidence with the aforesaid estimated reason.
Therefore, in order to solve the problem, desired is the increase in the strength of the interface between the resin and the release agent in a toner, namely, more specifically, desired is the increase in the adhesion at the interface between the resin and the release agent. For this purpose, it was found that incorporation of a polar group in both of resin and a release agent is effective to increase the affinity between the resin and the release agent, whereby the interface adhesion is improved. In this case, single use of a polar wax does not provide a fully satisfactory releasing property due to the compatibility of the wax with the polar resin. Therefore, it was found that preferable is a constitution in which a polar wax and a non-polar wax are utilized in combination in order to form a domain structure in a toner as well as to continuously increase the polar wax content from the inside to the out side in the interior of each domain. Techniques to distinctively utilize a polar wax and a non-polar wax are commonly known (for example, refer to Patent Documents 1-3), however, these are not to depress crush of a toner as will be described in the present invention, in which polar and non-polar waxes are utilized to improve the interface adhesion due to an interaction between the wax and the polar resin.
Patent Document 1 JP-A No. 11-149187 (JP-A refers to Japanese Patent Publication Open to Public Inspection)
Patent Document 2 JP-A No. 2000-267347
Patent Document 3 JP-A No. 2002-6542